The present invention relates to an optically-pumped external cavity surface emitting laser emitting single spatial and longitudinal mode radiation at selected wavelengths over a frequency comb. More particularly described are the laser design, manufacturing and assembly processes for optical fiber telecommunications.
Practical semiconductor lasers generally follow two basic architectures. The first laser type has an in-plane cavity, and the second laser type has a vertical cavity, a so-called vertical-cavity surface-emitting laser or xe2x80x9cVCSELxe2x80x9d. If the optical resonance cavity is formed externally of the semiconductor structure, the laser is known as a vertical external cavity surface-emitting laser or xe2x80x9cVECSELxe2x80x9d. Electrically pumped diode lasers are most frequently of the in-plane cavity type. Necessary optical feedback within the in-plane type is most frequently provided by simple cleaved-facet mirrors at each end of the optical cavity. The reflectance of such cleaved mirrors, while sufficient is not very high, and laser energy is emitted through the cleaved mirrors to the external ambient at opposed edges of the structure, giving rise to xe2x80x9cedge-emittingxe2x80x9d diode lasers. Such relatively simple structures are sometimes referred to as Fabry-Perot diode lasers. Epitaxial patterning of a grating pattern along a top surface of the edge-emitting diode laser is frequently provided to set a design wavelength, resulting in a distributed feedback diode laser or xe2x80x9cDFBxe2x80x9d.
In-plane electrically pumped (PIN diode) lasers, such as DFB lasers, are typically single mode, and are also typically tunable continuously across some wavelength band from near-infrared and into the visible light spectrum. Rapid tuning may be carried out by controlling the electrical pumping current, while slow tuning may be carried out by controlling the temperature of the laser via a heat sink and a thermal cooler/heater arrangement. In-plane lasers have many known uses including optical wavelength absorption spectroscopy, storage, printing and telecommunications. In-plane lasers are frequently employed within telecommunications systems using optical fiber as the information transfer medium. Conventionally, multiple channels are carried through a single optical fiber, and it is therefore necessary when using a Fabry-Perot diode laser or a DFB laser as the illuminating source to regulate the wavelength of the transmitting laser in order to stay on a selected channel.
In order to keep a diode laser tuned to a desired wavelength, complex current and thermal control loops must be provided to stabilize the laser at the desired wavelength, particularly as the laser ages during usage. Also, there is no absolute wavelength stabilization within these in-plane lasers, and the emission wavelength may drift, without careful feedback control, during usage and over the useful lifetime of the laser device. This tendency to drift or change characteristics with temperature and over time puts extremely stringent conditions on the materials used to make the laser.
One known drawback of in-plane diode lasers, and most particularly the Fabry-Perot type, is that it can manifest a tendency to mode-hop. Mode-hopping basically means that for a given pumping current, unexpectedly the laser can hop to a completely different mode (wavelength). As the current is increased, there are wavelengths at which the mode hopping or wavelength jumping becomes uncontrollable. Moreover, diode lasers may manifest a hysteresis, in that mode hopping may occur at different wavelengths during control current increases than the mod-hopping wavelengths encountered during control current decreases. Another drawback of in-plane diode lasers is that output power is inextricably intertwined with active region temperature and pumping current. Another issue with in-plane diode lasers is that the transverse optical beam profile is typically elliptical rather than circular and has high divergence, increasing the complexity of coupling the laser energy into the optical fiber, such as with precision gluing of tiny aspheric lenses at the laser-fiber interface.
Dense wavelength division multiplexing (DWDM) for optical fiber telecommunications applications require optical transmitters that can be tuned to any frequency in the standard ITU xe2x80x9cgridxe2x80x9d (wavelength comb) with a relative frequency error not greater than ten percent of the ITU channel spacing. This requirement implies that an optical transmitter laser has extreme frequency stability as well as broad tunability. For a 12.5 GHz channel spacing, the transmitter must have 1.25 GHz of absolute accuracy and frequency (wavelength) stability. Such control of the lasing frequency cannot be achieved with existing DFB lasers without complex electronic control and frequently carried out diagnostics. Furthermore, compensation algorithms must be developed in the laser control to handle the DFB""s known aging processes, which is often unpredictable.
Another requirement for an ideal DWDM optical transmitter is that a single laser can cover all of the DWDM channels, and that it can be reliably and reproducibly set to any one of the standard channel frequencies. Practically, a laser source will only have a limited tuning range, which covers only a fraction of the full ITU grid. Existing telecommunications DFB lasers have limited tunability; and, the temperature tuning coefficient of telecom DFB lasers is typically 0.09 nm/xc2x0 C. For a DFB laser thermal operating range of +20xc2x0 C., or 40xc2x0 C. total temperature differential, one DFB laser could only be expected to cover a wavelength range of 3.6 nm (or about 460 GHz, representing only four channel coverage with 100 GHz channel spacing or 36 channel coverage with 25 GHz spacing) provided that necessary accuracy in wavelength could be achieved.
In addition, DFB lasers only have about 30 to 35 dB of side mode suppression. If the side modes are not sufficiently controlled, the laser may excite two or three adjacent communications channels, resulting in unwanted interference. Because of these drawbacks, the telecommunications industry has recently turned to VCSELs.
Micro-cavity VCSELs include semiconductor structures which have multiple layers epitaxially grown upon a semiconductor wafer/substrate, typically Gallium Arsenide or Indium Phosphide. The layers comprise semiconductor or dielectric Bragg mirrors which sandwich layers comprising quantum well active regions. Within the VCSEL photons emitted by the quantum wells bounce between the mirrors and then are emitted vertically from the wafer surface. The VCSEL type laser naturally has a circular dot geometry with lateral dimensions of a few microns. The emitting aperture of a few microns facilitates direct-coupling to optical fibers or other simple optics, since the narrow aperture typically supports only a single lateral mode (TEM00) of the resulting optical waveguide, but is sufficiently wide to provide an emerging optical beam with a relatively small diffraction angle. Recently, a 1.3 micron VCSEL was said to be developed by Sandia National Laboratories in conjunction with Cielo Communications, Inc. According to a news report, xe2x80x9cThis new VCSEL is made mostly from stacks of layers of semiconductor materials common in shorter wavelength lasers . . . aluminum gallium arsenide and gallium arsenide. The Sandia team added to this structure a small amount of a new material, indium gallium arsenide nitride (InGaAsN), which was initially developed by Hitachi of Japan in the mid 1990s. The InGaAsN causes the VCSEL""s operating wavelength to fall into a range that makes it useable in high-speed Internet connections.xe2x80x9d (xe2x80x9cxe2x80x98First everxe2x80x991.3 micron VCSEL on GaAsxe2x80x9d, Optics.Org Industry News, posted June 16, 2000). One of the characteristics of micro-cavity VCSELs is that the laser cavity is formed entirely within the semiconductor structure. One drawback of such VCSELs is that they do not generate very much power, on the order of 3 mw for a small aperture of 5 xcexc, for example. Also, there is transverse spatial hole burning between the transverse modes above 3 mw, for example.
As mentioned above, if a cavity is formed which is external to the semiconductor structure having the quantum well active region, it is known as a VECSEL. A VECSEL epitaxially grown semiconductor body typically has a few microns thick multiple quantum well active gain region sandwiched between a Bragg mirror grown on a semiconductor substrate and an antireflection coating that is either epitaxially grown or dielectrically deposited. An external high reflectivity dielectric concave mirror is then added to form an external optical cavity One example of an optically pumped VECSEL is described in Published International Patent Application WO 00/10234, entitled xe2x80x9cOptically-Pumped External-Mirror Vertical-Cavity Semiconductor-Laserxe2x80x9d, the disclosure thereof being incorporated herein by reference. The disclosed VECSEL includes an epitaxially-grown semiconductor structure or chip having a multiple-layer mirror structure integrated with a multiple-layer quantum-well structure which provides a gain medium, and an external mirror forming a resonant cavity with the integrated semiconductor multilayer mirror. Optical pumping radiation is directed at the quantum-well structure via an outermost or top layer and is absorbed by the quantum-well and pump-absorbing layers. The quantum-well layers release photons in response to the pumping energy, and the external cavity is dimensioned to result in laser energy output at an approximate 976 nm wavelength in response to pumping energy at a wavelength of approximately 808 nm. Because this VECSEL operates at wavelengths below 1.1 xcexc in the visible light spectrum, the active gain medium is made to be aluminum-free, since aluminum ions tend to diffuse in GaAs materials-based lasers. Accordingly, the quantum-well and pump-radiation absorbing layers are aluminum-free layers of alloys of gallium arsenide and indium gallium arsenide phosphide (GaAs/InGaAsP). One drawback of the VECSEL described in this published International Patent Application is the absence of any wavelength tuning mechanism enabling adjustment of the laser emission wavelength.
Other VECSELs are described, inter alia, in a paper by Sandusky and Brueck, entitled: xe2x80x9cA CW External-Cavity Surface-Emitting Laserxe2x80x9d, IEEE Photonics Tech. Ltrs., Vol. 8, No. 3, March 1996, pp. 313-315; and, in a paper by Kuznetsov, Hamimi, Sprague, and Mooradian, entitled: xe2x80x9cHigh Power ( greater than 0.5-W CW) Diode-Pumped Vertical-External-Cavity Surface-Emitting Semiconductor Lasers with Circular TEM00 Beamsxe2x80x9d, IEEE Photonics Tech. Ltrs., Vol. 9, No. 8, August 1997, pp. 1063-1065.
Co-inventors Garnache and Kachanov of the present invention have previously reported in a note entitled xe2x80x9cHigh-sensitivity intracavity laser absorption spectroscopy with vertical-external-cavity surface-emitting semiconductor lasersxe2x80x9d, Optics Letters, Vol. 24, No. 12, Jun. 15, 1999, pp. 826-828, that an optically pumped multiple-quantum-well (xe2x80x9cMQWxe2x80x9d) VECSEL is an excellent candidate for use in high sensitivity intracavity laser absorption spectroscopy (xe2x80x9cICLASxe2x80x9d). In the ICLAS method an absorbent analyte is placed inside an external cavity of a broadband laser with homogeneously broadened gain. An L-shaped cavity was formed by the integrated Bragg mirror, an external folding mirror having a 150 mm radius of curvature, and a flat output coupler having 0.5 percent transmission placed at the end of a one meter arm of the cavity. The angle between the two arms was reduced to approximately 7 degrees to reduce astigmatism. A 500 mm long intra-cavity absorption cell with Brewster-angle windows and containing an analyte material was placed in the long arm. Generation time was controlled by an optical chopper that interrupts or starts the pump radiation beam and by an acousto-optic modulator that is triggered after an adjustable generation delay time. Further work by these authors with VECSELs in the field of spectroscopy is reported in a paper by Garnache, Kachanov, Stoeckel and Houdre entitled: xe2x80x9cDiode-Pumped Broadband Vertical-External-Cavity Surface-Emitting Semiconductor Laser: Application to High Sensitivity Intracavity Laser Absorption Spectroscopyxe2x80x9d, JOSA-B-B, Vol. 17, No. 9, September 2000, pp. 1589-1598. The disclosures of these two articles are incorporated herein in their respective entireties by this reference thereto.
An intra-cavity etalon and a Lyot filter were said by Holm et al. to stabilize VECSEL radiation at a single wavelength in xe2x80x9cActively Stabilized Single-Frequency Vertical-External-Cavity AlGaAs Laserxe2x80x9d, IEEE Photonics Technology Letters, Vol. 11, No. 12, December 1999.
As an alternative solution to DFB lasers for telecom, one approach for tuning a VECSEL is described in a note by D. Vakhshoori, P. Tayebati, Chih-Cheng Lu, M. Azimi, P. Wang, Jiang-Huai Zhou and E. Canoglu entitled, xe2x80x9c2 mW CW single mode operation of a tunable 1550 nm vertical cavity surface emitting laser with 50 nm tuning rangexe2x80x9d, published in Electronics Letters, Vol. 35, No. 11, May 27, 1999, pp. 900-901, the disclosure thereof being incorporated herein by reference. This laser design is grown epitaxially upon an indium phosphide substrate and has a cavity formed by a distributed Bragg reflector (DBR), a multiple quantum well (MQW) active gain region, and an external dielectric membrane mirror at a relatively short (xcx9c7 xcexc) distance from the active gain region. Because the VECSEL laser cavity is so short, only one cavity mode can fit into the bandwidth of the MQW gain structure. Cavity length can be changed by applying a potential difference between the dielectric membrane and the ambient supporting structure (heatsink), thereby applying an electrostatic force to the membrane mirror and causing its curvature (and hence the cavity length) to change. Changing the cavity length shifts the cavity resonance frequency which results in laser frequency tuning. The VECSEL is optically pumped by a 980 nm diode laser which can be epitaxially grown below the DWDM laser. The authors and an associated company, Coretek, have reported continuous tuning of this VECSEL over a range of about 50 nm, which is more than 10 times the tuning range of a typical DFB laser. This Coretek VECSEL is said to have a high quality TEM00 transverse mode and more than 50 dB of side mode suppression.
The Coretek VECSEL does not appear to meet the DWDM telecom requirements, however. First, the tuning voltage of 40 V that is needed for 50 nm wavelength tuning is too high for most telecom applications. Moreover, the micro-machined membrane mirror must be reasonably flexible in order to move the required tuning distance, and is necessarily sensitive to external perturbations or vibrations and also will become self-excited into undesirable vibrational modes by actuation. This system is very complex to produce, with evident difficulties of a multilayer epitaxial structure being compounded by the need to form, align and attach a precision micro-machined membrane external mirror. Thus, workable Coretek VECSELs would be very challenging to manufacture at a reasonable cost and yield in mass production. Furthermore, a complex feedback control system would be requried to maintain membrane mirror position, thereby limiting absolute frequency set point stability and reproducibility in laser tuning.
From the foregoing description of the state of the art, it is apparent that a hitherto unsolved need has remained for a simplified, reproducible and widely tunable single mode MQW VECSEL for optical fiber telecommunications in a manner overcoming the limitations and drawbacks of the prior approaches.
One object of the present invention is to realize an optical fiber transmitter module including a single mode MQW VECSEL having a semiconductor structure with a homogeneously broadened active gain region and an external mirror spaced from the semiconductor structure by a spacer such that a cavity length is in a range of 0.5 mm and 50 mm and is chosen to create a laser frequency comb corresponding to a predetermined optical channel spacing arrangement.
Another object of the present invention is to realize a MQW VECSEL semiconductor structure formed by molecular beam epitaxy in a manner enabling removal of the semiconductor substrate, thereby overcoming limitations and drawbacks of prior approaches in which the substrate contributed to the presence of a Fabry-Perot etalon or other unwanted optical element.
Yet another object of the present invention is to realize a laser with reproducible absolute wavelengths that correspond only to standardized wavelength division multiplex (WDM) channel wavelengths as used in optical fiber telecommunications networks, such that the laser steps from channel to channel and such that by design the absolute channel wavelengths of this laser are ensured to hit the channel wavelength exactly, that is have exact channel separation and exact channel wavelength with accuracy better than ten percent of channel spacing.
One more object of the present invention is to realize an optically pumped MQW VECSEL having sidemode suppression well in excess of 40 dB.
In accordance with aspects of the present invention, an optically pumped single mode MQW VECSEL includes a heat sink structure and a semiconductor structure grown by molecular beam epitaxy upon a substrate and attached to the heat sink. As completed, the semiconductor structure essentially comprises a multi-layer semiconductor mirror region, such as a Bragg mirror achieving at least 99 percent reflectance, a homogeneously broadened multiple quantum well active gain region having a length equal to at least one design wavelength and having a plurality of quantum wells, each quantum well being optimally positioned respect to a standing wave in the active gain region at the design wavelength, and an antireflection coating region having a low reflectance at the design wavelength. An external spherical mirror is positioned relative to the semiconductor structure by a spacer structure mounted to the heat sink at a distance in a range of 0.5 mm and 50 mm to form the external cavity and chosen to create a laser frequency comb corresponding to a predetermined optical channel spacing arrangement. Preferably, an in-plane laser (e.g. Fabry Perot or DFB laser diode) providing pump radiation is aligned relative to an external surface of the semiconductor at Brewster""s angle formed relative to an axis of laser emission. In this aspect the laser diode pump is a sub-assembly which is aligned and secured in a sidewall of the spacer and is thereby made integral therewith. A pump radiation absorption element or aperture is preferably formed in the spacer at Brewster""s angle opposite an angle of incidence of the pump radiation for absorbing any pump radiation residually reflected from the external surface of the semiconductor.
As another aspect of the present invention an optical fiber transmitter comprises an optically pumped single mode VECSEL for emitting an information-carrying laser beam at a design wavelength and has an external cavity length defining a plurality of optical modes, each mode corresponding to a channel wavelength of an optical telecommunications system having plural optical channels. A semiconductor structure of the VECSEL has an optical-pump-excited, homogeneously broadened multiple quantum well (MQW) active region wherein the gain curve exceeds cavity losses over a band which is less than mode-to-mode spacing, the gain region being tunable to step from a first mode to an adjacent second mode and to remain stably at the adjacent second mode. A tuning arrangement tunes the VECSEL from mode to mode thereby to select each one of the plural optical channels. A conventional optical amplitude modulator adds user traffic to a beam emitting from the laser to provide the information-carrying laser beam, and a coupler couples the traffic-carrying laser beam into an optical fiber of the optical telecommunications system.
A method for calibrating a VECSEL is provided. The VECSEL includes an external cavity formed by an external mirror at a length fixed by a spacer. The length defines a plurality of optical modes, each mode corresponding to a channel wavelength of an optical telecommunications system having plural optical channels. The semiconductor structure of the VECSEL has an optical-pump-excited, homogeneously broadened multiple quantum well (MQW) active region wherein the gain curve exceeds cavity losses over a band which is less than mode-to-mode spacing, the gain region being tunable to step from a first mode to an adjacent second mode and to remain stably at the adjacent second mode. A tuning mechanism such as a thermoelectric cooler for cooling and heating the active gain region, or a frequency-selective element such as an intra-cavity etalon, is provided for tuning the VECSEL from mode to mode. A digital controller including a wavelength-selective optical sensor responsive to VECSEL output radiation tunes the VECSEL from mode to mode and maintains the VECSEL at each mode thereby to select each one of the plural optical channels. The wavelength-selective optical sensor generates pulses responsive to inter-mode optical transitions. The method includes steps of:
sweeping the tuning mechanism between a longest wavelength mode capable of being generated by the VECSEL and a shortest wavelength mode capable of being generated by the VECSEL in accordance with a control parameter generated by the digital controller,
recording in a memory of the digital controller a transition control parameter presently being put out by the wavelength-selective optical sensing means upon detection of pulses responsive to inter-mode optical transitions, and
determining and recording single mode set values as approximately half increment magnitudes between magnitudes of adjacent recorded transition control parameters.